Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session R11: Convection and Buoyancy-Driven Flows: Experimental Studies |
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Chair: Yogesh Jaluria, Rugers University Room: 111 |
Tuesday, November 24, 2015 12:50PM - 1:03PM |
R11.00001: Experimental Investigation of Transport Enhancement in Convective Air Flow by the Use of a Vortex Promoter Yogesh Jaluria, Kevin Gomes This paper focuses on the effect of placing a passive vortex generator in a flow and the resulting increase in transport rates. The flow circumstance considered is that of a flat plate with protruding heat sources, placed in a uniform flow, with a vortex generator located upstream of the leading edge. The study consists of three parts. In the first part, the flow due to the vortex promoter by itself is considered. The periodic or chaotic behavior in the wake behind the promoter is investigated. By studying different sizes and shapes of vortex promoters, it is determined which configuration offers the largest disturbance in the flow and the frequency at which it occurs. In the second part of the study, the flow over a plate with isolated, finite-sized, protruding heat sources, without a vortex promoter, is considered. Again, the frequency of the disturbance downstream is investigated to determine the nature of the resulting flow and the disturbance frequency. The effect of varying the dimensions and locations of the heat sources on the flow downstream is investigated. It is found that a larger separation distance between two sources leads to higher transport rates. In the last part of the study, tests are done for the combination of vortex promoter and the plate, placing a vortex promoter in front of the plate. An effort is made to match the frequencies of the disturbances due to the vortex generator with those due to the plate in an attempt to achieve resonance. From these results, an optimal promoter is chosen that would lead to maximum heat transfer rate. [Preview Abstract] |
Tuesday, November 24, 2015 1:03PM - 1:16PM |
R11.00002: Transient Convection from Forced to Natural with Flow Reversal on a Vertical Flat Plate Blake W. Lance, Barton L. Smith Transient flow through the forced, mixed and natural convection regimes is studied experimentally on a vertical flat plate. Measurements are ensemble-averaged and include velocity from Particle Image Velocimetry and high fidelity thermal measurements in walls for both temperature and heat flux. The flow is a ramp-down flow transient encompassing all three convection regimes. The initial condition is forced convection downward with subsequent transition to mixed convection, ending with natural convection upward after a flow reversal. Velocity measurements provide time-mean and Reynolds stress profiles across the span of the test section. Near-wall data provide shear stress estimates by fitting a line to the data in the inner portion of the viscous sublayer. Wall heat flux is measured in the plate with thin film heat flux sensors. Flow reversal was observed near the heated plate and turbulence kinetic energy was redistributed from the heated boundary layer towards the freestream. Wall heat flux was decreased and shear stress decreased then reversed. These data are part of a CFD validation dataset meant to assess simulation accuracy and are the final case made available for mixed convection. [Preview Abstract] |
Tuesday, November 24, 2015 1:16PM - 1:29PM |
R11.00003: Physical modelling of LNG rollover in a depressurized container filled with water Petr Denissenko, Maksim Dadonau, Antoine Hubert, Siaka Dembele, Jennifer Wen Stable density stratification of multi-component Liquefied Natural Gas causes it to form distinct layers, with upper layer having a higher fraction of the lighter components. Heat flux through the walls and base of the container results in buoyancy-driven convection accompanied by heat and mass transfer between the layers. The equilibration of densities of the top and bottom layers, normally caused by the preferential evaporation of Nitrogen, may induce an imbalance in the system and trigger a rapid mixing process, so-called rollover. Numerical simulation of the rollover is complicated and codes require validation. Physical modelling of the phenomenon has been performed in a water-filled depressurized vessel. Reducing gas pressure in the container to levels comparable to the hydrostatic pressure in the water column allows modelling of tens of meters industrial reservoirs using a 20 cm laboratory setup. Additionally, it allows to model superheating of the base fluid layer at temperatures close the room temperature. Flow visualizations and parametric studies are presented. Results are related to outcomes of numerical modelling. [Preview Abstract] |
Tuesday, November 24, 2015 1:29PM - 1:42PM |
R11.00004: Exchange flow of two immiscible Newtonian fluids in a vertical tube Priscilla Varges, Fernanda Nascentes, Bruno Fonseca, Paulo Roberto de Souza Mendes, Monica Naccache Plug cementing is an essential operation performed under a variety of well conditions. The cement plugs are rarely placed at the intended depth because the cement slurry usually is heavier than the well fluid. Failures are due primarily to migration of the denser fluid downward through the drilling fluid at the top of which it is discharged. The aim of the research is to better understand the process of plugging operation in vertical wells. To this end, we performed an experimental and theoretical study of the buoyancy-driven flow of two immiscible Newtonian fluids in a vertical tube such that the heavier and more viscous fluid is placed on top. Since both fluids are Newtonian, the situation is always unstable, i.e. the fluid on top will always flow downward and displace the bottom fluid upwards, so that the relative positioning tends to invert. The influence of the governing parameters on the speed of inversion was investigated. Flow visualization was performed with a digital camera, and inversion velocities were obtained through image analysis. Preliminary results show that inversion speed decreases as the tube diameter is increased, increases as the viscosity ratio is increased, and also increases as the density ratio is increased. [Preview Abstract] |
Tuesday, November 24, 2015 1:42PM - 1:55PM |
R11.00005: A cryostat device for liquid nitrogen convection experiments Charles Dubois, Alexis Duchesne, Herve Caps When a horizontal layer of expansible fluid heated from below is submitted to a large vertical temperature gradient, one can observe convective cells. This phenomenon is the so-called Rayleigh-B\'enard instability. In the literature, this instability is mainly studied when the entire bottom surface of a container heats the liquid. Under these conditions, the development of regularly spaced convective cells in the liquid bulk is observed. Cooling applications led us to consider this instability in a different geometry, namely a resistor immersed in a bath of cold liquid. We present here experiments conducted with liquid nitrogen. For this purpose, we developed a cryostat in order to be able to perform Particle Image Velocimetry. We obtained 2D maps of the flow and observed, as expected, two Rayleigh-B\'enard convective cells around the heater. We particularly investigated the vertical velocity in the central column between the two cells. We compared these data to results we obtained with silicone oil and water in the same geometry. We derived theoretical law from classical models applied to the proposed geometry and found a good agreement with our experimental data. [Preview Abstract] |
Tuesday, November 24, 2015 1:55PM - 2:08PM |
R11.00006: The shape and behaviour of a horizontal buoyant jet adjacent to a surface Henry Burridge, Gary Hunt We investigate the incompressible turbulent buoyant jet formed when fluid is steadily ejected horizontally from a circular source into a quiescent environment of uniform density. As our primary focus, we introduce a horizontal boundary. For sufficiently large source-boundary separations, the buoyant jet is `free' to rise under the action of the buoyancy force. For smaller source-boundary separations, the jet attaches and `clings' to the boundary before, further downstream, pulling away from the boundary. Based on measurements of saline jets in freshwater we deduce the conditions required for a jet to cling. We present data for the variation in volume flux, flow envelope and centreline for both `clinging' and `free' jets. For source Froude numbers $\textrm{fr}_{0} \geq 12$ the data collapses when scaled, identifying universal behaviours for both clinging jets and for free jets. [Preview Abstract] |
Tuesday, November 24, 2015 2:08PM - 2:21PM |
R11.00007: Dissolution patterns on caramel blocks Caroline Cohen, Julien Derr, Michael Berhanu, Sylvain Courrech du Pont We investigate erosion by dissolution processes. We perform laboratory experiments on hard caramel bodies, which dissolve on a short timescale, compared to geological material such as limestone. We put a block of caramel, tilted from the horizontal, in a water tank without flow. The dissolution syrup, which is denser than pure water, sinks and the flow detaching from the surface creates patterns underneath the caramel block. These patterns result from the coupled dynamics of the flow detaching and the eroding surface and are reminiscent of scallops observed in the walls of phreatic cave passages. We investigate the mechanisms of formation of these structures and their evolution depending on several parameters such as the fluid density or the flow velocity. We finally parallel the formation of patterns on melting iceberg. [Preview Abstract] |
Tuesday, November 24, 2015 2:21PM - 2:34PM |
R11.00008: Spatio-temporal intermittency in stratified shear flow: effects of Prandtl number Adrien Lefauve, Paul Linden We present laboratory experiments of a stratified shear flow in an inclined square duct, connecting two reservoirs of water at different densities. The exchange flow in which a layer of dense salt-water flows beneath a layer of lighter fresh-water moving in the opposite direction is known to host a rich zoo of behaviors. As the driving density difference or inclination angle is increased, the initially sharp and flat density interface can support Holmboe waves, and then transition to a statistically steady turbulent intermediate mixed layer. Here we report on the significantly different dynamics observed when stratification is achieved by heat instead of salt. Even moderate values of density difference or inclination angle now allow the dramatic growth of interfacial waves causing the flow to transition to a fully-turbulent state, before it relaxes back to a laminar state and so forth. These novel laminar-turbulent cycles exhibit a remarkable periodicity and suggest that the Prandtl number ($Pr=700$ for salt vs $Pr=7$ for heat) affects the intermittency and mixing properties of stratified turbulence. [Preview Abstract] |
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